Utah wildlife officials are scrambling to determine what led to the death of four bald eagles in northern Utah in the last week. The eagles all appeared healthy, with the exception of head tremors, but eventually died. Three were delivered to the Wildlife Rehabilitation Center of Northern Utah in Ogden and another to Great Basin Wildlife Rescue in Mapleton. (Courtesy Wildlife Rehabilitation Center of Northern Utah, The Salt Lake Tribune)

It is that characteristic – specifically of finding carrion on the water – that has led to the death of at least 21 bald eagles and impacted many more across northern Utah.

State wildlife officials announced Tuesday morning that the National Wildlife Health Center in Madison, Wis. confirmed West Nile virus is the cause of the eagle deaths and that six more are currently being treated in wildlife rehabilitation centers.

West Nile virus was suspected when sick and dead eagles started to show up in early December, but wildlife officials were reluctant to identify the mosquito-born virus as the culprit because it has never been reported in December.

“This is a major development. This is not typically a time we would expect to see West Nile virus. We monitor West Nile every year, but during mosquito season. This is an unusual event,” said Leslie McFarlane, wildlife disease specialist for the Utah Division of Wildlife Resources (DWR). “It is concerning and something we will have to watch for going into the future.”

As the number of eagle deaths grew, the investigation turned to the ice-free waters of the Great Salt Lake. Eared grebes, a species of migratory waterfowl found in western North America, gather on the lake in massive numbers, approximately 2 million, each fall and linger until their food sources diminish before continuing south.

It is common for large numbers of the grebes to die each fall, usually from avian cholera. McFarlane said it appears that up to 20,000 grebes died this year and the bald eagles quickly honed in on the easy meal.

“We became more suspicious when the eagles were showing up pretty much centered around the Great Salt Lake,” McFarlane said. “We are not sure West Nile is killing the grebes, but they are definitely a carrier of it.”

If it ends up that the eared grebes died from West Nile virus it would mark a first time deaths have recorded in that species. The National Wildlife Health Center is performing tests on the grebes.

“People become infected with West Nile virus after being bitten by a mosquito that carries the virus,” said JoDee Baker, epidemiologist with the Utah Department of Health in a release. “Although there are other very rare ways you can get the virus, such as receiving contaminated blood or organs from an infected person, mosquitoes are, by far, the most common method of transmission. Since the mosquitoes that transmit West Nile virus aren’t active in the winter, there’s no risk to the public’s health.”

McFarlane said an estimated 10,000 grebes remain on the Great Salt Lake, but are likely close to flying south.

“As long as we have live grebes that are sick out there and infected dead birds that eagles will eat we may continue to lose bald eagles,” she said.

Efforts to clear the Great Salt Lake of the dead grebes would be expensive and a tremendous undertaking. Wildlife biologists believe the West Nile virus does not remain in the carcasses of dead animals very long.

“It doesn’t do well in the environment without a host,” McFarlane said.

DaLyn Erickson with the Wildlife Rehabilitation Center of Northern Utah in Ogden has treated nine of the bald eagles. Five of them died or had to be euthanized. All were suffering from head tremors, lower body paralysis and an inability to digest food.

Erickson started treating four eagles in her care with anti-inflammatories last week and it appears to be working.

While she is glad to finally have a name for what is impacting the eagles, Erickson is not happy with the prognosis.

“I wish there was a cure for West Nile, but there is not,” she said. “We are doing all we can to help them, but it is basically up to the bird’s immune system if they can fight it off. All we can do is help them with that fight.”

Three of the birds being treated at the Wildlife Rehabilitation Center of Northern Utah are now able to stand and are eating. A juvenile male that arrived at the center in a more compromised state is not faring as well, but has shown signs of improvement.

“West Nile does damage to the brain, the heart and other organs,” Erickson said. “We need to see if that damage is permanent or if it will heal in each individual case.”

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